3GPPthird generation partnership projectUTRANuniversal terrestrial radio access networkEUTRANevolved UTRAN (LTE)LTElong term evolutionNode Bbase stationeNBEUTRAN Node B (evolved Node B)UEuser equipmentULuplink (UE towards eNB)DLdownlink (eNB towards UE)DRXdiscontinuous receptionEPCevolved packet coreMMEmobility management entityS-GWserving gatewayMMmobility managementPHYphysicalRLCradio link controlRRCradio resource controlMACmedium access controlPDCPpacket data convergence protocolO&Moperations and maintenanceFDDfrequency division duplexTDDtime division duplexHARQhybrid automatic repeat requestCQIchannel quality indicatorSRSsounding reference signalOFDMAorthogonal frequency division multiple accessSC-FDMAsingle carrier, frequency division multiple accessTTItransmission time intervalRTTround trip timePDCCHphysical downlink control channel
A communication system known as evolved UTRAN (EUTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. As presently specified the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.3.0 (2007-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8), incorporated by reference herein in its entirety.
FIG. 7 reproduces FIG. 4 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1MME interface and to a Serving Gateway (SGW) by means of a S1U interface. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:
functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);
IP header compression and encryption of the user data stream;
selection of a MME at UE attachment;
routing of User Plane data towards the Serving Gateway;
scheduling and transmission of paging messages (originated from the MME);
scheduling and transmission of broadcast information (originated from the MME or O&M); and
measurement and measurement reporting configuration for mobility and scheduling.
Another document of more specific interest herein is 3GPP TS 36.321 V8.2.0 (2008-05) Technical Specification, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification (Release 8), which is incorporated by reference in its entirety. Subclause 5.7, entitled Discontinuous Reception (DRX), states that the UE may be configured by RRC with a DRX functionality that allows it to not continuously monitor the PDCCH.
Another document of interest is 3GPP TSG RAN WG1 #52 Meeting, R1-080958, Sorrento, Italy, Feb. 11-15, 2008, “HARQ Numbers of LTE TDD considering the proposal of special subframe”, Nokia, Nokia Siemens Networks, which is also incorporated by reference herein.
As is specified in 3GPP TS 36.321, the DRX functionality consists of a Long DRX cycle, a DRX Inactivity Timer, a DRX Retransmission Timer and optionally a Short DRX Cycle and a DRX Short Cycle Timer, all defined in Subclause 3.1.
When a DRX cycle is configured, the Active Time includes the time while:                the On Duration Timer or the DRX Inactivity Timer or a DRX Retransmission Timer or the Contention Resolution Timer is running; or        a Scheduling Request is pending (as described in subclause 5.4.4); or        an uplink grant for a retransmission can occur; or        a PDCCH indicating a new transmission addressed to the C-RNTI or Temporary C-RNTI of the UE has not been received after successful reception of a Random Access Response (as described in subclause 5.1.4).        
When a DRX cycle is configured, the UE shall for each subframe:                start the On Duration Timer when [(SFN*10)+subframe number] modulo (current DRX Cycle)=DRX Start Offset;        if a HARQ RTT Timer expires in this subframe and the data in the soft buffer of the corresponding HARQ process was not successfully decoded:        start the DRX Retransmission Timer for the corresponding HARQ process.        if a DRX Command MAC control element is received:        stop the On Duration Timer;        stop the DRX Inactivity Timer.        if the DRX Inactivity Timer expires or a DRX Command MAC control element is received in this subframe:        if the short DRX cycle is configured:                    start the DRX Short Cycle Timer and use the Short DRX Cycle.                        else:        use the Long DRX cycle.        if the DRX Short Cycle Timer expires in this subframe:        use the long DRX cycle.        during the Active Time, for a PDCCH-subframe except if the subframe is required for uplink transmission for half-duplex FDD UE operation:        monitor the PDCCH;        if the PDCCH indicates a DL transmission:        start the HARQ RTT Timer for the corresponding HARQ process;        stop the DRX Retransmission Timer for the corresponding HARQ process.        if the PDCCH indicates a new transmission (DL or UL):        start or restart the DRX Inactivity Timer.        if a DL assignment has been configured for this subframe and no PDCCH indicating a DL transmission was successfully decoded:        start the HARQ RTT Timer for the corresponding HARQ process.        when not in active time, CQI and SRS shall not be reported.        
Regardless of whether the UE is monitoring PDCCH or is not monitoring the PDCCH it receives and transmits HARQ feedback when such is expected.
It is noted that the concept of the DRX Retransmission Timer is related to the operation of a HARQ process by way of a HARQ RTT Timer.
That is, the DRX Retransmission Timer specifies the maximum number of consecutive PDCCH subframe(s) during which a DL retransmission is expected by the UE, while the HARQ RTT Timer specifies a minimum number of subframe(s) before a DL HARQ retransmission is expected by the UE.
The joint operation of the DRX Retransmission Timer and the HARQ RTT Timer, in the case of a new transmission, is currently specified as was described above.
Certain rules for a downlink HARQ process are that at most one HARQ process is configured to one UE in one DL subframe, and one HARQ RTT Timer and one DRX Retransmission Timer per DL HARQ process, when DRX is active, are configured.
Since there are both DL and UL transmissions in each subframe in FDD, it can be specified that the HARQ RTT Timer is always 8 TTIs in FDD, as shown in FIG. 1A. Due to the characteristics of FDD it may be concluded that HARQ processes which begin at different subframes are independent, as there is no collision of the starting points of the DRX Retransmission Timer.
Discussing now is a problem that arises in the TDD mode. Since there is either a DL or an UL transmission in each subframe in TDD (not both as in the case of the FDD mode), the RTT values of the HARQ process beginning at each subframe in one frame are different. Reference in this regard can be made to the Table shown in FIG. 1B, which is reproduced from the above-referenced R1-080958 document.
Under the present operational rules the start of retransmission of different HARQ processes can potentially collide in a DL subframe. FIG. 2 shows an example of a 2DL+1S+2UL configuration. In FIG. 2D represents a DL subframe, U represents an UL subframe, and S represents a special subframe.
General reference with regard to UL, DL and S subframes for the TDD mode can be made to Subclause 4.2, Frame structure type 2, of 3GPP TS 36.211 V8.3.0 (2008-05) Technical Specification, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8), incorporated by reference herein in its entirety.
It can be observed in FIG. 2 that without HARQ process 1, the retransmission of HARQ process 2 can begin in subframe 12, while with HARQ process 1, the retransmission of HARQ process 2 may be delayed to subframe 15. Accordingly, with HARQ process 2, the retransmission of HARQ process 3 may be delayed to subframe 16.
A more extreme configuration, 8DL+1S+1UL, is depicted in FIG. 3. In this case up to 9 HARQ processes may collide. As a result, problems can arise in designing a DRX Retransmission Timer and HARQ RTT Timer for each process. For example, if the DRX Retransmission Timer expiry value is made small, such as 2 DL subframes, the window for monitoring the PDCCH is not sufficient when most or all possible HARQ processes exist. Further, if the DRX Retransmission Timer expiry value is made large, such as 9 DL subframes, the effect of power saving is reduced in the case where only a few HARQ processes exist (e.g., only processes 1 and 9). Further still, if the HARQ RTT Timer value is set according to the maximum RTT among all subframes, the delay is increased for those subframes whose RTT is small in the case where only a few HARQ processes exist.